The guys here have vastly more experience tuning carbs than I do, but here's my basic core dump:

First, what's your baseline setting? What's your home altitude? If it's a used bike, especially, don't trust the specs to be accurate. If you haven't already, take apart the carb and figure out what jets, needle, and clip setting you are running presently. How does it work for you? Good throttle response off-idle? Does it rev out as you need it to? Is your air filter clean? Do you have a small supply of spark plugs in the proper size and range? Get an assortment of jets in the sizes close to your baseline setting, and get an extra clip or two (DAMHIK ).

Increase in altitude will tend to make the engine run rich--less oxygen for the same amount of fuel. When you get to the event, as Lineaway says, take a little time to check operation. Then make a small change if you need to. After the change, use a clean plug. Warning: it's much harder to read a trials plug compared to a racing engine because you don't operate WFO. If the plug is dark and wet your setting is too rich on one or more of the circuits, especially if the engine stumbles when you change the throttle setting. If the plug is lighter than a nice cinnamon tan you may be too lean. If the bike runs hot or runs on after closing the throttle that can be a sign of leanness.

Generally speaking, adjust pilot for idle, needle/clip for mid, main for larger throttle settings. They will interact to some degree but try to make one change at a time and target performance changes you want.

Now is the time for guys with more trials-specific carb experience to chime in. Is there a better way to deal with circuit interaction?